Electric induction apparatus

ABSTRACT

562,902. Transformers. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 29, 1943, No. 1565. Convention date, Feb. 3, 1942. [Class 38 (ii)] In an ignition transformer in which an interrupted metallic shield 21 is placed between the windings to prevent the electrostatic transmission of radio frequency energy from the secondary to the primary circuit, a continuous sheet of conducting material 25 forming a shortcircuited turn about the core also is interposed between the windings to prevent the transmission of radio frequency energy by electromagnetic induction. The sheet 25 is made very thin (.4 of a mil.) so that it offers high impedance to current of the supply frequency ; it may be a sheet of copper foil on an insulating backing 27, held in position against the primary winding 10 by tape 28. Specifications 369,889, [Group XXXVI], and 534,149 are referred to.

Jan. 11, 1944. M. o. MARSH I ELECTRIC INDUCTION APPARATUS Filed Feb. s, 1942 Ir-lvef-wtr Myr'le @.Mafsh, .b my 6.411.414

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Patented Jan. 1l, 19d4 g 2,339,088 ELECTRIC INDUCTION APPARATUS Myrle 0. Marsh, Fort Wayne, Ind., assignor to rGeneral Electric Company,

New York a corporation of Application February 3, 1942, Serial No. 429,382

7 claims. (o1. irs- 356) My invention relates to electric induction apparatus and to transformers which when employed in certain applications are susceptible of providing a path through which relatively high Vfrequency impulses which cause radio interferencek may pass.

Transformers ofthe type which provide igni-V tion for oil burner equipment have a secondary winding-connected to electrodes across which the ignition arc passes, and a primary winding connected to a source of supply. It is known that upon operation of the transformer to produce a spark at the electrodes, high frequency impulses are produced at the sparking electrodes which circulate through the secondary high voltage winding and which if allowed to pass tothe primary winding would cause impulses to pass through the wiring to the house source of supply, causingk radio disturbances to surrounding radios. In order to prevent these high voltage impulses from passing from the secondary to the primary windings it has been the practice to provide a shield between the windings, the shield being provided with a discontinuity so as to prevent it from forming a complete and continuous short-circuited turn around the winding leg in which currents of the fundamental frequency may circulate. It has been found, however, that such a shield although helping to prevent high frequency impulses from being transmitted from the secondary winding to the primary winding, nevertheless still allows a suiiicient amount of energy of the high frequency value to be transmitted to the primary Winding so as to cause disturbance to surrounding radios. In order to prevent this disturbance, it has been suggested to use suitably designed capacitors in the primary circuit, these capacitors being designed to lter oil the radio frequency impulses not absorbed by' the shield. These capacitors, however, are expensive and have the additional disadvantage that they appear to be responsible for high surge voltages in the primary supply line when the the burner control shuts off the ignition. Such inductive kicks appear to be magnified by the capacitor so that high voltages may be obtained and trouble with other household appliances.

It is, therefore, an object of my invention to provide a transformer with an improved arrangement for preventing impulsesrof radio frequency values vfrom being transferred to the primary winding.'

Another object of my invention is to provide an improved transformer structure which will keep radio interference to a minimum and which will be simple in design and economical to manufacture. Y

A further object of my invention is to provide an electric induction apparatus with an improved arrangement for preventing high frequency impulses from passing through the apparatus.

Further objects and advantages of my invention will become apparent from the following description -referring to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a partof this specification.

In the arrangement illustrated in the drawing, I have provided a shielding arrangement for a transformer having a metallic shield of magnetic or non-magnetic material which is placed between the primary and secondary windings, the shield having a discontinuity therein so as to prevent the vcompletion of a continuous shortcircuited path around the winding leg. In order `which cause excessive incandescent lamp failures c vto prevent the transfer of energy of the high frequency or rawdio frequency type from the secondary to the primary winding which is not absorbed by the conventional shield, I provide a closed loopA of conducting material between the primary and secondary windings. This conductive loop also has such characteristics that it will provide a relatively high impedance to circulating currents induced by the fundamental or 60-cycle frequency voltage. v

In the drawing Fig. 1 is a sectional side elevation of a transformer provided with an embodiment of my invention; Fig. 2 is an exploded view thereof; and Fig. 3 is a graph'showing a comparison between a shielded and a non-shielded transformer. f

Referring more particularly to Fig. l of the drawing, I have illustrated a transformer which includes a primary .or low voltage winding II) and a high voltage Winding which includes a pair of sections II and I2. Lines I3 and I4 which are connected at one end to the low voltage or primary winding I0 are adapted to be connected to any suitable source of supply, and load lines I5 and VII are adapted to be connected to a suitable load such as the electrodes of an oil burner ignition apparatus. Although this transformer to which my improved shielding arrangement is applied has .particular application for ignition, it is to be understood that my invention may be employed with any other suitable transformer or electric induction apparatus. The transformer also employs the conventional core I'I which has nected to the secondary winding through the conductors I5 and I6, impulses of the high frequency type which will cause radio interference are produced at the spark gap, and these high frequency impulses will partially pass through the secondary winding sections II and I2. These impulses, unless the proper shielding is employed, will in turn cause impulses of similar frequencies to be transferred to the primary winding I0 which impulses may pass through the conductors I3 and I4 causing disturbances in surrounding radios. In order to prevent this it is customary to provide a shield 2J between the high and low voltage windings. As will be seen in Fig. 2 of the drawing, this shield 2l has an aperture 22 for receivingthe winding leg I8 and apertures 2 3 for receiving the outer yoke portions of the core. A discontinuity 24 is provided so as to prevent circulating currents from being produced in the shield 2|. Such a shield 2I when placed between the primary and secondary windings, as is illustrated in Fig. l, will have found, however, that even with the conventional shield 2I sufficient impulses are transferred in some manner, such as by magnetic induction, to the primary winding I0 so as to cause radio interference to nearby radios.

In order, therefore, to prevent the transfer of high frequency energy from the secondary-winding to the primary winding which transfer is not prevented by the conventional shield, I provide in addition to the shield 2l a closed loop of conducting material between the primary and secondary windings and coaxial therewith. This loop may be of any suitable type and in the arrangement illustrated in the drawing it includes a relatively thin conducting material 25' ondary windings but I have found it convenient to place it close to the primary winding which has a relatively' low voltage, so as to facilitate Y the insulating of the conducting sheet. Also, the conducting sheet may be supported in any suitable manner and inthe arrangement illustrated in the drawing the conducting sheet 25 is supported by a sheet of suitable insulating material 21 such as paper, the contour of the insulating sheet 2l being substantially the same as that of the conducting material 25. A few wrappings `28 of suitable insulating material may be employed for supporting the conducting sheet 25 with its insulating backing 2l on that face of the primary winding which faces the secondary winding portions II and I2. Either the insulating sheet or the conducting side 25 may be placed adjacent the insulating covering of the primary winding II). The conducting material 25 should have such electrical characteristics so that it bination of the high reactance and high resistance gives a sufficient impedance so as to minimize the flow of -cycle or fundamental frequency current in the closed loop 25. The conducting loop 25 may be composed of any suitable material and size so as to have these desired characteristics and in the arrangement shown in the drawingit is composed of a thin sheet of copper of approximately .4 of a mil in thickness. 'I'his relatively thin copper foil may be formed in any suitable manner such as by electrolytically depositing the copper on a. roll and then removing it and mounting it on the insulating material 21.

I have found that when my short-circuited turn is employed with the conventional shield substantially all radio interference is eliminated. It appears that there is some high frequency electromagnetic induction taking place between the secondary and primary windings and the closed turn absorbs the energy which would .otherwise be transferred by this electromagnetic induction. The conventional shield 2|, it is felt, is more ofthe electrostatic type and tends to absorb the high frequency flux which tends to take passage through the air from the high voltage coil to the low voltage coil. The high voltage coil has a very high inherent impedance to the flow of high frequency energy, and virtually all the energy is transferred from the high voltage side of the transformer to the low voltage side by means of an air path. It is my belief that there i s a small amount of energy of the electromagnetic type as distinguished from the electrostatic type which tends to transfer itself either along the surface of the magnetic core or by direct electromagnetic induction in the coil itself. A transfer of energy by this electromagnetic induction is substantially prevented by the short-circuited turn since this turn absorbs that energy and prevents it from passing to the primary winding IIJ. 'I'he thin shield also has a relatively high impedance so that normal low frequency impulses induced therein are a minimum.

In order to illustrate the substantial improvement in preventing radio interference with a. transformer employing my invention, I have diagrammatically illustrated in Fig. 3 a comparison of the amount of radio interference which would be caused with a transformer core and coils as illustrated in Fig. 1 if no shielding were provided, if only the shield 2I were provided, and when my improved short-circuited turn in combination with the shield 2| is employed. In the graph I have plotted the amount ol.' radio interference in microvolts as ordinates, and the frequency in megacycles of the high frequency impulses in the secondary winding as abscissa, the ordinates being in rectangular coordinates and the abscissa in logarithmic coordinates. 'I'he tests were made in each case with similar core and coils,

, the amount of radio interference being measured will absorb substantially all the high frequency energy which is not filtered out by the conven tional shield 2I and it should still have a sulficiently high resistance to the passage of the 25 relatively close to the primary winding I 0 ithas a relatively high reactance and the comby' a standard interference meter which is well knownin the art. In each case the frequency was varied over the normal broadcast and high frequency bands and the microvolts determined. The curve identified by the numeral 30 shows the amount of radio interference which was ob tained when no shielding was provided between the primary and secondary windings. Curve 3| illustrates the amount of radio interference which obtains when the conventional shield 2| is employed, and the curve represented by the numeral 32 illustrates the substantial improvement which is obtained when my improved shielding construction is employed which includes the conventional shield and the short-circuited turn 25. Such an improvement as will be seen in the drawing is particularly marked in the lower frequency or broadcast band, and where interference is particularly obnoxious to the usual home receiver.

In view of the foregoing it will be seen that I have provided an improved arrangement for preventing the transfer of high frequency impulses between the windings of an electric induction device such as a transformer which includes a,

conventional shield of magnetic or non-magnetic material having a relatively high conductivity and a discontinuity so as to prevent circulating currents and a closed loop having a relatively high resistance to the passage of normal power frequency impulses but which will absorb high frequency impulses which tend to be induced in the primary winding.

Modifications of the particular ,arrangement which I have disclosed embodying my invention will occur to those skilled in the art, so that I do not desire my invention to be limited to the particular arrangement set forth and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of my invention.

What I claim as new and desire to secure b Letters Patent of the United States is:

1. In a transformer, primary and secondary windings, said secondary winding adapted to have impulses of radio frequency values susceptible of being transferred to said primary winding, and means for shielding said primary winding including a metallic shield between said windings, said shield having a discontinuity so as to prevent circulating currents from being induced therein, and a. second shield of conducting material forming a closed loop between said windings and co' axial therewith for substantially preventing the passage of high frequency impulses from the secondary to primary which are not stopped by said shield, said conducting material having a relatively high impedance to relatively low frequency impulses induced1 therein.

2. In a transformer, a core of magnetic material having a winding leg, primary and secondary windings surrounding said winding leg, said secondary winding being adapted to have impulses of radio interferences upon operation of the transformer susceptible of being transferred to said primary winding, and means for shielding said primary winding including a metallic shield disposed around said winding leg and between said windings, said shield having a discontinuity so as to preventy the completion of a continuous short-circuited turn around said winding leg, and a second shield of conducting material 4between said windings and forming a closed loop around said winding leg for absorbing energy of the high frequency type, said second shield having a relatively high impedance to relatively low frequency energy so as to maintain the low voltagefrequency impulses induced therein to a minimum.

3. In a transformer, primary and secondary windings, said secondary winding being adapted to have impulses of radio frequency values upon operation of the transformer susceptible of being transferred to said primary winding, and means for shielding said primary winding including a metallic shield between said windings, said shield having a discontinuity so as to prevent the completion of a continuous short-circuited turn, and a conducting material between said shield and said primary winding and coaxial therewith forming a closed loop for absorbing high frequency energy, said conducting material providing a relatively high impedance to relatively low frequencies.

4. In a transformer, primary, and secondary windings, said secondary winding being adapted to have impulses of radio frequency values upon operation of the transformer susceptible of being transferred to said primary winding, and means for shielding said primary winding including a metallic shield between said windings, said shield having a discontinuity so as to prevent the completion of a continuous short-circuited turn, and a second shield of relatively thin sheet of conducting material between said windings and forming a closed loop coaxial therewith for ablsorbing energy of the high frequency type, said conducting material having a relatively high impedance torelatively low frequency energy.

5. In an electric induction apparatus, primary and secondary windings, said secondary winding being adapted to have impulses of radio frequency values upon operation of" the transformer susceptible of being transferred to said primary winding, and means for shielding said primary winding including a metallic shield between said windings, said shield having a discontinuity so as to prevent the completion of a continuous shortcircuited turn, and an insulating sheet having a relatively thin covering of conducting material, said sheet and conducting material being disposed between said windings and coaxial therewith for absorbing energy of the high frequency type.

6. In an electric induction apparatus, a core having a winding leg, primary and secondary windings surrounding said leg, said secondary winding being adapted to have impulses of radio frequency values upon operation of the transformer susceptible of being transferred to said primary winding, and means for shielding said primary winding including a metallic shield between said windings, said shield having a discontinuity so as to prevent the completion of a continuous short-circuited turn, an insulating sheet having a relatively thin covering of conducting material, and means for supporting said sheet and conducting material on the side of said primary winding facing said secondary winding. 7. In an electric induction apparatus, a core of magnetic material having a winding leg, primary and secondary windings surrounding said winding leg, said secondary winding being adapted to have impulses of radio frequencies upon operation of the transformer susceptible of being trans-u ferred to said primary winding, and means for shielding said primary winding including a metallic shield disposed around said winding leg and between said windings, said shield having a discontinuity so as to prevent the completion of a continuous short-circuited turn around said winding leg, an insulating sheet having a thin covering of conducting material on a face thereof, said sheet having substantially the same contour as said windings, and means for supporting said shield on a face of said primary winding so that high frequency energy will be absorbed by said conducting material.

MYRLE O. MARSH. 

